Field emission displays include an anode and a cathode structure. The cathode is configured into a matrix of rows and columns, such that a given pixel can be individually addressed. Addressing is accomplished by placing a positive voltage on one row at a time. During the row activation time, data is sent in parallel to each pixel in the selected row by way of a negative voltage applied to the column connections, while selected pixels on the anode are held at a high positive voltage. The voltage differential between the addressed cathode pixels and the anode pixels accelerates the emitted electrons toward the anode.
Color field emission display devices typically include a cathodoluminescent material overlying an electrically conductive anode. The anode resides on an optically transparent frontplate and is positioned in parallel relationship to an electrically conductive cathode. The cathode is typically attached to a glass backplate and a two dimensional array of field emission sites is disposed on the cathode. The anode is divided into a plurality of pixels and each pixel is divided into three subpixels. Each subpixel is formed by a phosphor corresponding to a different one of the three primary colors, red, green, and blue. Correspondingly, the electron emission sites on the cathode are grouped into pixels and subpixels, where each emitter subpixel is aligned with a red, green, or blue subpixel on the anode. By individually activating each subpixel, the resulting color can be varied anywhere within the color gamut triangle. The color gamut triangle is a standardized triangular-shaped chart used in the color display industry. The color gamut triangle is defined by each individual phosphor's color coordinates, and shows the color obtained by activating each primary color to a given output intensity.
So long as the pixels are sufficiently large, relative to a given electron beam size, the color gamut available at the frontplate of the display is only limited by color output of a given phosphor. Under ideal operating conditions, electrons emitted by the addressed emitter subpixels on the cathode only strike the intended subpixel on the anode. However, in many practical systems of interest, such as high-voltage displays, the beam width of the emitted electrons is not confined to a particular subpixel on the anode. At the relatively large cathode to anode separation distances used in high voltage displays, the electron beam spreads and stray electrons can strike adjacent subpixels on the anode. This phenomenon is know as "color bleed." As the color bleed increases, the available color gamut of the display is decreased.
In addition to color bleed, misalignment of the anode to the cathode can degrade the color performance of a display. Any misalignment can cause some subpixels to receive a higher than intended electron beam intensity, while others receive a diminished electron beam intensity. Even a slight amount of misalignment shifts the color coordinates of each phosphor and can result in a reduction of the color gamut available from the display.
To overcome the loss of color gamut, switched anode techniques in combination with frame sequential addressing have been developed. A switched anode provides separate circuits for subpixels of the same color, but located in adjacent pixels. The groups of subpixels on the anode are electrically connected to form two separate networks. An electronic control system is provided for sequentially addressing alternating rows and columns of pixels on the anode and on the cathode. Adjacent pixels are assigned an odd or even designation in order to separate the activation of the same color subpixels located in adjacent pixels on the anode. This approach significantly improves color performance, but at the expense of additional complexity and cost in the fabrication of the electronic circuitry necessary to operate the display.
Another method used to overcome color bleed is to add additional electrodes in the cathode to focus the emitted electron beam. The electron beam spreading is controlled by electrostatically confining the electron beam, such that the beam strikes the intended subpixel on the anode.
While the switched anode techniques and additional focusing structures improve color performance, these can be difficult to implement in a high voltage display and they require more complicated electronics, which add to the expense of the display. Furthermore, additional processing steps are often necessary, which increase the manufacturing cost of the display. Accordingly, a need existed for a low-cost, color field emission display having improved color performance.